1. Field of the Invention
The present invention relates to an ink jet recording device, and specifically to an ink jet recording device capable of reliably providing high quality images at a high printing speed.
2. Related Art
There has been proposed a line scanning type ink jet printer, capable of printing images on an elongated uncut recording sheet at a high printing speed. This type of printer includes a head having a plurality of nozzles and an elongated width covering over the entire width of the recording sheet. When printing images, ink droplets are ejected from the nozzles based on a recording signal and alight or impact on the recording sheet that is being fed at a high speed in its longitudinal direction. By controlling both the ink ejection and the feed of the recording sheet, a desired image is provide on the recording sheet.
There are two types of line scanning type ink jet printer. One includes a continuous ink jet head, and the other includes an on-demand ink jet head. Although the printer with the on-demand ink jet head is slow in printing speed compared with the printer with the continuous ink jet head, the on-demand ink jet head requires a simple ink system, and so is well suited for general-purpose high-speed printers.
An on-demand ink jet head of a line-scanning type ink jet printer is formed with a plurality of nozzle lines, each including a plurality of nozzles aligned in a line. Each of the nozzles includes an ink chamber and is provided with an energy generating member, such as a piezoelectric element or a heat generating element. When a driving voltage is selectively applied to the energy generating member, ink in the ink chamber is applied with pressure, and some of the ink is ejected as an ink droplet through a nozzle hole.
The present inventors have invented an ink jet printer including such ink jet head and, in addition, charger/deflector electrodes. The charger/deflector electrodes charge an ink droplet ejected from the nozzle and also generate a deflector electric field that deflects the charged ink droplets in flight, thereby controlling a position on the recording sheet to alight or impact (hereinafter referred to as xe2x80x9cimpact positionxe2x80x9d). In this type of ink jet printer, a plurality of ink droplets ejected from different nozzles can be controlled to alight on the same single spot to form a single dot on the recording sheet. Because each dot on the recording sheet is formed from a plurality of ink droplets from different nozzles, even if one or ones of the different nozzles become defective, the dot is still formed by the remaining nozzle(s). Therefore, images can be formed reliably. Also, because each dot is formed by a plurality of different nozzles, bands of darker or lighter gray tones and lines on the printed image due to uneven characteristics among the plurality of nozzles can be canceled out, and so a high quality image, without uneven color density or a white line across the page, can be provided.
U.S. Pat. No. 5,975,683 discloses an electrically insulated steering electrode positioned near the nozzle hole. The steering electrode steers a charged droplet in a desired direction when charged with a voltage. There is also disclosed other type of steering electrode, which is positioned behind a recording sheet, rather than near the nozzle hole. These electrodes could be used in principle as the above charger/deflector electrode.
However, a conventional deflector electrode is insufficient in its operational reliability or in its deflecting capability. Specifically, an electrically insulated deflector electrode provided near the nozzle hole may get wet with ink. This unstabilizes a generated deflector electric field and prevents a desirable deflection control. Also, electrically insulated deflector electrode may be deteriorated in its insulating performance when get wet with ink, inhibiting a deflector voltage from being applied to the deflector electrode. The insulating performance will also be deteriorated due to chemical reaction, such as oxygenation, carbonization, and the like.
On the other hand, a deflector electrode provided behind a recording sheet is relatively far away from the nozzle hole, so that there is generated only a deflector electric field that cannot effectively deflect charged ink droplets in flight. Accordingly, a sufficient deflection amount cannot be provided.
It is an object of the present invention to provide a highly reliable charger/deflector device that generates a deflector electric field capable of effectively deflecting charged ink droplets from an early flight stage.
It is also an object of the present invention to provide a charger/deflector device that includes a deflector electrode with an excellent deflection performance and that is well suited for an ink droplet deflection type on-demand ink jet printer.
In order to achieve the above and other objectives, there is provided a charging/deflecting device used in a device including a nozzle member that is formed with an ink chamber filled with an ink and an ejection means for ejecting a portion of the ink as an ink droplet. The charging/deflecting device includes an electrically conductive member, a back electrode, and an application member. The electrically conductive member is provided near a position where the ink droplet is generated by separating from the remaining ink at the time of ejection, and has the same potential as that of the ink filling in the ink chamber. The back electrode is positioned defining a space between the electrically conductive member through which a recording medium passes. The application member applies an electric voltage to the back electrode, thereby generating an inclined electric field between the electrically conductive member and the back electrode.
There is also provided an ink jet recording device including a nozzle member that is formed with an ink chamber filled with an ink, an ejection means for ejecting a portion of the ink as an ink droplet, an electrically conductive member, a back electrode, and an application member. The electrically conductive member is provided near a position where the ink droplet is separated from the remaining ink at that time of ink ejection. The electrically conductive member has the same potential as the potential of the ink filling in the ink chamber. The back electrode is positioned defining a space between the electrically conductive member through which a recording medium passes. The application member selectively applies an electric voltage to the back electrode, thereby generating an inclined electric field between the electrically conductive member and the back electrode.